Furnace for high-frequency heating with the aid of oscillations of very high frequency



P 28, 1965 J. VERSTRATEN ETAL 3,209,113

FURNACE FOR HIGH-FREQUENCY HEATING WITH THE AID 0F OSCILLATIONS OF VERY HIGH FREQUENCY Filed May 18, 1962 INVENTOR JAN VERSTRATEN FRANCISCUS J. H. TIMMER MANS AGEN United States Patent 3,209,113 FURNACE FOR HIGH-FREQUENCY HEATING WITH THE AID 0F OSCILLATION'S OF VERY HIGH FREQUENCY Jan Verstraten and Franciscus Josephus Hendricus Timmermans, Eindhoven, Netherlands, assignors to North American Philips Company, Inc., New York, N.Y., a corporation of Delaware Filed May 18, 1962, Ser. No. 195,798 Claims priority, application Netherlands, July 17, 1961, 267,183 15 Claims. (Cl. 219 10.55)

The invention relates to a furnace for high-frequency heating of substances by means of electromagnetic energy of very high frequency, for example, energy in the decimeter or centimeter range. This furnace comprises a very high frequency generator and a waveguide system bounded by two parallel closing Walls. The output circuit of the very high frequency generator is connected to the waveguide system for guiding the very high frequency energy to the outlet port of the waveguide system along which the objects to be heated are conveyed in a direction which is substantially at right angles to the two parallel closing walls of the waveguide system.

In a prior U.S. Patent 3,102,181 an advantageous highfrequency heating furnace of the kind set forth above is described, which is characterized in that the two parallel closing walls are bounded on the side remote from the outlet port by a parabolic cylinder surface. In the focal line of the parabolic cylinder surface provision is made of a linear radiator. Between the linear radiator and the outlet port of the Waveguide system remote from the peak of the parabola a reflector is arranged so as to reflect the direct radiation emanating from the linear radiator and directed towards the outlet port of the waveguide system. Throughout the width of the outlet port, which, may for example, be 120 cms. in a practical embodiment of a highfrequency heating furnace with a Wavelength of the oscillations produced of 12 cms., a uniform heating is obtained in this device and the stray radiation emanating from the waveguide system is materially reduced.

In practice the device described yielded excellent results, but it has been found that, when the dimensions of the waveguide system were reduced, for example, in order to reduce the dimensions of the outlet port and to raise the concentration of power thereat, practical difliculties arose. Applicants have found that with such a high-frequency heating furnace the advantages of the uniform heating and the attenuation of the stray radiation projecting beyond the waveguide system Were disturbed, which was found by experiments to be due to the fact that the reduction of the dimensions adversely affects the satisfactory operation of the radiation system of the heating furnace formed by the linear radiator and the reflector due to the surroundings, particularly the objects to be heated.

The invention 'has for its object to provide a different structure of a device of the kind set forth, in which, while the advantages of uniform heating and reduced stray radiation are maintained, an appreciably greater freedom in the choice of the dimensions of the high-frequency furnace is obtained, so that apart from a mechanically rugged structure a marked increase in power concentration at the outlet port can be realized.

The device according to the invention is characterized in that the two parallel boundary Walls of the waveguide system are bounded in the plane remote from the outlet port by a semi-parabolic cylindrical surface, the generatrices of which are at right angles to the parallel boundary walls, and by a boundary surface of waveguide horn, arranged at the side of the outlet port of the waveguide system in the focal line of the semi-parabolic cylindrical surface, which horn radiates very high-frequency oscillations from the ultrahigh-frequency generator towards the semi-parabolic surface. The horn is furthermore bounded by the parallel boundary surfaces of the Waveguide system and by a boundary surface leading to the outlet port and intercepting direct radiation from the waveguide horn towards the outlet port of the waveguide system.

The invention and its advantages will now be described more fully with reference to the accompanying drawing wherein:

FIG. 1 shows in a perspective view one embodiment of a high-frequency heating furnace according to the invention.

FIG. 2 is a cross sectional view of the high-frequency heating furnace of FIG. 1 and FIG. 3 shows a further development of the device according to the invention.

The high frequency heating furnace shown in a perspective view in FIG. 1, having a power of, for example, 5 kw., comprises a magnetron generator 1 which is capable of producing oscillations having a wavelength of, for example, 12 cms. The output circuit of the magnetron generator 1 is connected via a coaxial conductor 2 and by means of a rectangular waveguide 3 with a waveguide system 6. Waveguide system 6 is bounded by two parallel boundary walls 4, 5, for guiding the oscillations produced towards the outlet port 7 of the waveguide system 6. The boundary walls 4, 5 are partly omitted in order to show the interior of the waveguide system 6. Along the outlet port 7 of the waveguide system 6 are conveyed the objects 8 to be heated, for example, deepfrozen meals, in a direction at right angles to the two boundary walls 4, 5 via conveyer belt 9, which is driven by driving rollers (not shown). For adaptation to the load of the magnetron generator the rectangular waveguide 3 has connected therewith the tuning means 10, which may be constructed in known manner.

In order to obtain a uniform heating throughout the outlet port 7 of comparatively small width with a high power concentration in the high-frequency heating furnace so far described, the two parallel boundary walls 4, 5 are closed in the plane remote from the outlet port 7 by a semi-parabolic cylindrical surface 11, the generatrices of which are orthogonal to the parallel boundary surfaces and by a boundary surface 12 of a waveguide horn 13, arranged at the side of the outlet port 7 of the waveguide system 6 in the focal line of the semi-parabolic cylindrical surface 11, which horn radiates the oscillations from the magnetron generator 1 towards the parabolic cylindrical surface. The waveguide horn is bounded by the parallel boundary surfaces 4, 5 of the waveguide system 6 and a boundary surface 14 leading to the outlet port and intercepting direct radiation from the waveguide horn 13 towards the outlet port 7. In the device described above the output circuit of the magnetron generator 1 is coupled with the waveguide 3 by means of a linear radiator 15, which is arranged parallel to the generatrices of the semiparabolic cylindrical surface 11 and hence at right angles to the axis of the parabola. The waveguide 3 is connected via a coupling piece 16 with the opening of the waveguide horn 13. If desired, the magnetron generator 1 may be coupled in a different manner with the waveguide 3, for example, by a loop coupling.

In order to obtain a closed structure, the waveguide system is connected with a pass channel 17 of conductive material surrounding the conveyor belt 9. At the area of the outlet port 7 of the waveguide system 6 the pass channel 17 is bent over in a direction away from the conveyor belt 9 and shaped in the form of a rectangular trough 18, which is partly filled with absorbing material 20 (see FIG. 2) for absorbing the radiation from the magnetron generator 1 passing the conveyor belt 9, so

walls 4, 5 of the waveguide system 6. The opening of the waveguide horn 13, measured from the center of the radiation mouth, may be about 120.

Substantially the complete radiation from the waveguide horn 13, emitted in accordance with the radiation characteristic curve described, is reflected from the semiparabolic cylindrical surface 11 of the waveguide system 6 towards the axis of the parabola to the outlet port 7 of the waveguide system 6, so that at the outlet port 7, in directions parallel to the boundary walls 4, 5, a cophase, electromagnetic field of constant intensity is obtained and the objects 8 to be heated, passing on the conveyor belt 9 by the outlet port 7 of the waveguide system 6 in a direction at right angles to the boundary walls "4, 5 are uniformly heated.

In the device described the direction of radiation of the waveguide horn 13 is directed away from the outlet port 7. With the structure of the high-frequency furnace described the waveguide horn is particularly directed towards the semi-parabolic cylindrical surface and it has been found that a direct coupling of the objects to be heated with the waveguide horn 13 is thereby considerably reduced. Without adversely affecting the uniform electromagnetic field produced at the area of the outlet port 7, the dimensions of the waveguide system 6 can then be considerably reduced, for instance in order to reduce the width of the outlet port 7 in order to raise the power concentration at this area, which is illustrated in the cross sectional view of FIG. 2.

With the high-frequency furnace of 5 kw. described above, the height of the waveguide system 6, measured from the conveyor belt 9, is 30 cms. and the width of the outlet port 7 is 30 cms. The distance between the two boundary walls 4, 5 is 4.3 cms. The power concentration at the outlet port 7 amounts to 42 w./cm.

Apart from the rugged, compact structure and the uniform heating of the objects to be heated, the device described has the practically important advantage that the radiation projecting beyond the high-frequency furnace is considerably reduced. As stated above, the direction of the electric field vector E of the field irradiated by the linear radiator 15 is parallel to the generatrices of the semi-parabolic cylindrical surface 11 and therefore it has a course at the outlet port 7 of the waveguide system 6 indicated in FIG. 1 by the broken arrows 19. The energy vector (Poynting vector) is at right angles tothe electric field vector E and has therefore substantially no component in the pass direction of the conveyor belt 9, so that from the waveguide system 6 only stray radiation can emanate, which is, moreover, attenuated in the pass channel 17, for example, by a factor of about 26 db. In the embodiment described the length of the pass channel 17 is 28 cms. and the height 5 cms.

Summarizing it can be stated that the high-frequency heating furnace according to the invention has a rugged mechanical structure and a freedom of dimensions and, moreover, produces a uniform heating while the stray radiation projecting beyond the waveguide system 6 is considerably attenuated.

FIG. 3 shows a further development of the high-frequency heating furnace according to the invention, in which elements corresponding with those of FIG. 1 are designated by the same reference numerals, to which an accent is added.

In this device use is made of the properties of the high- .device shown in FIG. 1.

4- frequency heating furnace described with reference to FIG. 1 for providing an improvement consisting in that two waveguide systems 6' and 6" with the associated magnetron generators 1' and 1" are connected with a common pass channel 17, surrounding the conveyor belt 9 and made of conductive material. Owing to the low stray radiation in the direction of the pass channel 17, the magnetron generators 1' and 1" are substantially decoupled, so that a relative reaction or destruction of the magnetron generators 1' and 1" need not be feared under varying operational conditions. In this case the waveguide systems 6 and 6 may even be arranged with the walls in contact with each other on the pass channel 17. With doubling of the power of the high-frequency furnace the stray radiation projecting beyond the pass channel 17 is found to be substantially the same as that of the In the high-frequency furnace concerned the maximum power of the stray radiation outside the furnace at the area of the operator is found to be less than 8 mw./cm.

In the embodiment shown in FIG. 3 the two waveguide systems 6' and 6 are arranged at a given distance from each other, so that time periods with high-frequency heating and without high-frequency heating alternate, which is advantageous for various uses, for example, for high-frequency heating of frozen meals from 20 to a temperature of C. In the time period without heating the heat produced during the preceding period is capable of spreading over the object 8 to be heated, which is conducive to a uniform heating, since owing to local variations of the dielectric constant of the objects 8 to be heated local irregularities in the high-frequency heating may occur. In the embodiment concerned this is, for example, the case at the thawing instants of the frozen meals.

With the device described the uniformity of the heating was further improved by arranging, in the first place, the waveguide horns 13' and 13" of the waveguide systems each on one side of the pass channel 17 and, furthermore, by connecting the two waveguide systems 6 and 6 in opposite senses with the common pass channel 17, the

latter measure being of particular importance for heating the objects 8 from both sides with a small depth of penetration of the ultrahigh-frequency radiation.

Apart from the improvements in uniformity of heating the high-frequency furnace has an important increase in efficiency, since the objects 8 to be heated pass by the outlet ports 7' and 7 of the waveguide systems 6' and 6" in different heating stages, so that these objects constitute different loads for the magnetron generators 1' and 1". By adapting each of these magnetron generators 1 and 1" separately with the aid of the adjusting means 10' and 10" to the object, an improvement in efiiciency of, for example, 30% is obtained.

Thus by carrying out the measures according to the invention, a high-frequency heating furnace is obtained which is distinguished by a considerable increase in efi'iciency and an improvement in the heating process and which exhibits a considerable increase in output power (10 kw.) and a minimum stray radiation.

What is claimed is:

1. High frequency heating apparatus comprising a high frequency generator and a waveguide system, said waveguide system comprising an enclosure having an open end and comprising first and second substantially parallel spaced apart wall portions, a third wall portion having a substantially parabolic contour arranged at right angles to and bounding said first and second wall portions and extending substantially from one end of the said open end to a predetermined point of said first and second wall portions remote from said open end, said parabolic portion defining a focal line at a position at one side of said open end, energy radiating means disposed at said focal line and arranged to radiate the high frequency energy towards said parabolic surface, means for coupling said generator to said radiating means, a fourth wall portion bounding said first and second Wall portions and extending between said radiating means and said predetermined point, a fifth wall portion forming a boundary surface extending between said radiating means and said open end and arranged to intercept the portion of direct energy radiation emanating from said radiating means towards said open end of said enclosure, and means for supporting objects to be heated at said open end of said enclosure.

2. High frequency heating apparatus comprising a high frequency generator and a waveguide system, said waveguide system comprising an enclosure having an open end and comprising first and second substantially parallel spaced apart wall portions, a third wall portion having a substantially parabolic contour arranged at right angles to and bounding said first and second wall portions and extending substantially from one end of the said open end to a predetermined point of said first and second wall portions remote from said open end, said parabolic portion defining a focal line positioned to one side of said open end, waveguide radiating means arranged to radiate the high frequency energy towards said parabolic surface and located in said focal line, means for coupling said energy from said generator to said waveguide means, a fourth wall portion bounding said first and second wall portions and extending between said waveguide radiating means and said predetermined point, said fourth wall portion forming an extension of one wall of said waveguide radiating means, a fifth wall portion extending between said radiating means and said open end, and means for supporting objects to be heated at said open end of said enclosure.

3. High frequency heating apparatus comprising a high frequency generator and a waveguide system, said waveguide system comprising first, second, third and fourth wall portions defining an enclosure having an open end, said first wall portion comprising a portion of a parabolic cylindrical surface which defines a focal line located at one side of said open end, said second and third wall portions forming two parallel boundary surfaces enclosing the ends of said parabolic surface and substantially at right angles thereto, and said fourth wall portion forming said enclosure with said first, second and third wall portions, waveguide means for radiating the high frequency energy towards said parabolic surface and located substantially in the focal line of said parabolic cylindrical surface, means for coupling said energy from said genera tor to said waveguide means, said fourth wall portion forming an extension of one wall of said waveguide means, said waveguide system further comprising a fifth wall portion extending between the radiating end of said waveguide means and the open end of said waveguide system so as to intercept the direct energy radiation from said waveguide means towards said open end, and means for supporting objects to be heated at said open end of said enclosure.

4. High frequency heating apparatus comprising a high frequency generator and a waveguide system, said waveguide system comprising first, .second, third and fourth wall portions defining an enclosure having an outlet port, said first wall portion comprising a semi-parabolic cylindrical reflecting surface, said second and third wall portions comprising two spaced parallel plates enclosing the ends of said parabolic surface and substantially at right angles thereto, and said fourth wall portion bounding said second and third Wall portions and forming said enclosure with said first, second and third wall portions, waveguide means having an open end facing said parabolic surface and located at the side of the outlet port of said waveguide system and in the focal line of the parabolic cylindrical surface, means for coupling said generator to said waveguide means, said fourth wall portion extending between said waveguide means and one end of said parabolic surface, a fifth wall portion extending between the open end of said waveguide means and said outlet port, and conveyor means for carrying objects to be heated past said outlet port of the waveguide system.

5. Apparatus as described in claim 4 wherein the walls of said waveguide means are bounded by said fourth wall portion, by said second and third parallel wall portions and by said fifth wall portion.

6. High frequency heating apparatus comprising a high frequency generator and a waveguide system, said waveguide system comprising first, second, third and fourth wall portions defining an enclosure having an outlet port, said second and third wall portions forming two parallel boundary surfaces of said waveguide system, said parallel boundary surfaces being bounded along a portion thereof by said first wall portion comprising a semiparabolic cylindrical surface having generatrices which are at right angles to said parallel boundary surfaces and along another portion thereof by said fourth wall portion, a waveguide horn located to the side of said outlet port and in the focal line of the parabolic cylindrical surface, said fourth wall portion forming a boundary surface of said waveguide horn which extends to one end of said parabolic surface, waveguide means for coupling the high frequency energy from said generator to said waveguide horn, said waveguide means being positioned so that the electric field vector of the electromagnetic energy supplied to said waveguide horn is parallel to the generatrices of said semi-parabolic cylindrical surface, said waveguide horn being further bounded by said second and third parallel boundary walls of the waveguide system and by a boundary surface extending from said horn to said outlet port and intercepting the direct energy radiation from said waveguide horn to said outlet port, and conveyor means for carrying objects to be heated past said outlet port. in a direction substantially at right angles to said second and third wall portions.

7. Apparatus as described in claim 6 wherein said high frequency generator is coupled to said waveguide means by a linear radiator extending within said waveguide means in a direction parallel to the generatrices of said semi-parabolic cylindrical surface thereby to supply to said waveguide horn electromagnetic waves in the TE mode having a direction of polarization parallel to the generatrices of said semi-parabolic cylindrical surface.

8. Apparatus as described in claim 6 wherein the radiation opening of the waveguide horn measured from the center of the radiation mouth is approximately degrees.

9. Apparatus as described in claim 6 further comprising means for enclosing said outlet port comprising a rectangular trough composed of a conductive material and providing aligned openings in said second and third wall portions, a radiation absorbing material disposed in said enclosing means, said conveyor means comprising a conveyor belt member extending through said aligned openings for supporting objects to be heated at said outlet port, and shield means enclosing said conveyor belt at portions thereof adjacent said aligned openings.

10. High frequency heating apparatus comprising high frequency generator means and a plurality of waveguide systems, each of said waveguide systems comprising an enclosure having an open end and comprising first and second substantially parallel spaced apart wall portions, a third wall portion comprising a semi-parabolic cylindrical reflecting surface arranged at right angles to and bounding said first and second wall portions and extend ing substantially from one end of the said open end to a predetermined point of said first and second wall portions remote from said open end, said parabolic portion defining a focal line positioned to one side of said open end, waveguide means for radiating the high frequency energy towards said parabolic surface and located to one side of said open end and substantially in the focal line of said parabolic cylindrical surface, means for coupling said energy from said generator to said waveguide means, a fourth wall portion bounding said first and secondwall portions and extending between said waveguide radiating means and said predetermined point, said fourth wall portion forming an extension of one wall of said waveguide radiating means, a fifth wall portion forming a boundary surface extending between said waveguide radiating means and said open end and arranged to intercept the direct energy radiation from said waveguide radiating means directed towards said open end of the enclosure, conveyor means for carrying objects to be heated past each of said openings, and a conductive hollow channel member disposed about said conveyor means and interconnecting said waveguide systems, said hollow channel member having a plurality of spaced apertures, each of said openings of said waveguide systems being associated with one of said apertures.

11. Apparatus as described in claim 10 wherein said waveguide means associated with different ones of said waveguide systems are mounted on different sides of said conveyor means.

12. Apparatus as described in claim 11 wherein said hollow channel member comprises at least first and second boundary surfaces disposed on opposite sides of said conveyor means and having at least one aperture in each of said boundary surfaces, predetermined ones of said waveguide systems being arranged on different sides of said conveyor means so that each of said openings of said waveguide systems is associated with a predetermined one of said apertures.

13. Apparatus as described in claim 12 wherein a first and second one of said waveguide systems are arranged in side by side relationship with one parallel boundary wall of said first waveguide system in contact with one of the parallel boundary walls of said second waveguide system.

14. Apparatus as described in claim 12 wherein said Waveguide systems are spaced apart along the length direction of said conveyor means by predetermined distances sufficient to allow the heat developed in said objects in the preceding waveguide system to be uniformly distributed in said objects during its travel between successive waveguide systems.

15. High frequency heating apparatus comprising a high frequency generator and a waveguide system, said waveguide system comprising first, second, third and fourth wall portions defining an enclosure having an outlet port in a given plane, said first wall portion comprising a portion of a parabolic cylindrical surface which defines a focal line located at one side of said open end, said second and third wall portions forming two parallel boundary surfaces enclosing the ends of said parabolic surface and substantially at right angles thereto, and said fourth wall portion forming said enclosure with said first, second and third wall portions, waveguide means for radiating the high frequency energy towards said parabolic surface and located substantially in the focal line of said parabolic cylindrical surface, means for coupling said energy from said generator to said waveguide means, said waveguide system further comprising a fifth wall portion forming a boundary surface extending between the radiating end of said waveguide means and the open end of said waveguide system, said fifth wall portion extending parallel to the plane of said outlet port and spaced inwardly from said plane in the interior of said waveguide system thereby to intercept the direct energy radiation from said waveguide means towards said outlet port, and means for supporting objects to be heated at said open end of said enclosure.

References Cited by the Examiner UNITED STATES PATENTS 3,027,442 3 62 Verstraten 219l0.55 3,102,181 8/63 Verstraten 219l0.55

- FOREIGN PATENTS 600,101 3 48 Great Britain. 1,255,297 1/ 61 France.

OTHER REFERENCES Electronics Magazine, McGraw Hill, June 10, 1960, page 47.

RICHARD M. WOOD, Primary Examiner. 

1. HIGH FREQUENCY HEATING APPARATUS COMPRISING A HIGH FREQUENCY GENERATOR AND A WAVEGUIDE SYSTEM, SAID WAVEGUIDE SYSTEM COMPRISING AN ENCLOSURE HAVING AN OPEN END AND COMPRISING FIRST AND SECOND SUBSTANTIALLY PARALLEL SPACED APART WALL PORTIONS, A THIRD WALL PORTION HAVING A SUBSTANTIALLY PARABOLIC CONTOUR ARRANGED AT RIGHT ANGLES TO AND BOUNDING SAID FIRST AND SECOND WALL PORTIONS AND EXTENDING SUBSTANTIALLY FROM ONE END OF THE SAID OPEN END TO A PREDETERMINED POINT OF SAID FIRST AND SECOND WALL PORTIONS REMOTE FROM SAID OPEN END, SAID PARABOLIC PORTION DEFINING A FOCAL LINE AT A POSITION AT ONE SIDE OF SAID OPEN END, ENERGY RADIATING MEANS DISPOSED AT SAID FOCAL LINE AND ARRANGED TO RADIATE THE HIGH FREQUENCY ENERGY TOWARDS SAID PARABOLIC SURFACE, MEANS FOR COUPLING SAID GENERATOR TO SAID RADIATING MEANS, A FOURTH WALL PORTION BOUNDING SAID FIRST AND SECOND WALL PORTION AND EXTENDING BETWEEN SAID RADIATING MEANS AND SAID PREDETERMINED POINT, A FIFTH WALL PORTION FORMING A BOUNDARY SURFACE EXTENDING BETWEEN SAID RADIATING MEANS AND SAID OPEN END AND ARRANGED TO INTERCEPT THE PORTION OF DIRECT ENERGY RADIATION EMANATING FROM SAID RADIATING MEANS TOWARDS SAID OPEN END OF SAID ENCLOSURE, AND MEANS FOR SUPPORTING OBJECTS TO BE HEATED AT SAID OPEN END OF SAID ENCLOSURES. 